1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine (hereinafter referred to as an engine) equipped with a failure detection device for an exhaust gas recirculation control system (EGR system), and more particularly, it relates to a new technique that is capable of preventing the false detection of failure of such an EGR system.
2. Description of the Related Art
In the past, in engine control apparatuses mounted on motor vehicles or the like, there have been proposed a variety of exhaust gas recirculation (EGR) control devices in which a part of an exhaust gas is recirculated into an intake pipe of an engine so as to reduce the combustion temperature of the engine thereby to suppress NOx components in the exhaust gas (see, for instance, a first patent document (Japanese patent application laid-open No. H8-28364) and a second patent document (Japanese patent application laid-open No. H8-35449)).
FIG. 8 is a constructional view that schematically illustrates the entire system of a general control apparatus for an internal combustion engine as described, for example, in the first patent document or the second patent document.
In FIG. 8, an EGR control device includes an engine 1, an air cleaner 2, an intake pipe 3, an intake manifold 4, an injector 5, a pressure sensor 6 for detecting the pressure Pb (negative pressure) in the intake pipe 3, a throttle valve 7 for controlling the amount of intake air Qa, a throttle opening sensor 8 for detecting the throttle opening θ of the throttle valve 7, a bypass air amount control section (ISC solenoid) 9, an air flow meter 10 for detecting the amount of intake air Qa, an EGR tube 11, an EGR valve 12 disposed in the EGR tube 11, an EGR opening sensor 13 for detecting the degree of opening θe of the EGR valve 12 (EGR opening θe), an angle sensor 14 for detecting the rotational angle of the engine 1, an exhaust pipe 15, a catalyst 16 disposed in the exhaust pipe 15 for purifying the exhaust gas flowing therein, a water temperature sensor 17, an idle switch 18, an air conditioning switch 19 for generating an air conditioner turn-on signal A, an air conditioner controller 19A for driving an air conditioner by the air conditioner control signal D, a battery 20, an ignition key switch 21, and an electronic control unit 22.
The pressure sensor 6, the throttle opening sensor 8, the EGR opening sensor 13, the angle sensor 14, the water temperature sensor 17, the idle switch 18, the air conditioning switch 19 and so on together constitute a sensor part that provides the operating condition information of the engine 1.
The electronic control unit 22 takes in the operating condition information (the throttle opening θ, the degree of EGR opening θe, an engine rotation angle rad (the number of revolutions per minute of the engine Ne), an idle signal I, the intake pipe pressure Pb, the amount of intake air Qa, a cooling water temperature Tw, the air conditioner turn-on signal A, etc.) from the sensor part, and controls the amount of fuel to be injected from the injector 5, the flow rate of EGR gas Qe (hereinafter referred to simply as the flow rate of EGR Qe), the amount of bypass air Qb, and the operation of the air conditioner in accordance with the operating condition information.
That is, the electronic control unit 22 includes a fuel control section related to the injector 5, an EGR control section related to the EGR valve 12, and an EGR failure determination section, and outputs a fuel injection control signal J to the injector 5, an EGR control signal C to the EGR valve 12, a bypass control signal B to the bypass air amount control section 9, an air conditioner control signal D to the air conditioner controller 19A, etc.
In the known device illustrated in FIG. 8, the electronic control unit 22 maintains a demanded output (a target number of revolutions per minute) during idling (throttle closed) operation by controlling the amount of bypass air Ob by means of the bypass air amount control section 9. In addition, when the cooling water temperature Tw is below a predetermined temperature (engine cooling state), or when the engine is loaded, e.g., during the operation of the air conditioner, the engine operates to ensure the demanded output by increasing the amount of intake air Qa under the control of the bypass air amount control section 9.
The amount of bypass air Qb changes in accordance with the intake pipe pressure Pb that is changed by the number of revolutions per minute of the engine Ne, etc., even if the passage area of the ISC solenoid (or the throttle valve 7) of the bypass air amount control section 9 is constant.
Moreover, for example, if the air conditioner turn-on signal A indicates an on (turn-on) command and the operating condition satisfies an air conditioner turn-on permission condition, an engine load driving section in the electronic control unit 22 generates an air conditioner control signal D for actuating the air conditioner, whereas if the air conditioner turn-on signal A indicates an off command, the engine load driving section generates an air conditioner control signal D for deactuating the air conditioner, whereby the turn-on or actuation of the air conditioner is controlled so as to give priority to ensuring the operating condition of the engine while preventing an excessively large load thereon.
However, the required flow rate of EGR gas might not sometimes be obtained due to defective operation of the EGR valve 12, the accumulation of exhaust gas deposits on the EGR valve 12 and/or the EGR tube 11 in accordance with the age of use. If such a state is left, there will arise a problem that a large amount of NOx continues to be generated, but such an abnormal flow rate of EGR of the exhaust gas can not be easily found or recognized by the driver.
Accordingly, as a failure detection device for such an EGR control section (EGR system), there has been proposed one that makes a failure determination based on an amount of change (pressure difference ΔP) in the intake pipe pressure in accordance with the change of the amount of intake air Qa due to the presence and absence of EGR when the EGR valve 12 is forced to open and/or close.
However, when the EGR valve 12 is opened and/or closed during the steady-state operation of the engine 1, the torque generated by the engine 1 is varied, thereby deteriorating the driveability of the vehicle.
Accordingly, in the above-mentioned first and second patent documents, the generated torque of the engine 1 is eliminated to prevent the deterioration of drivability by opening and/or closing the EGR valve 12 when the engine operating condition is a fuel cut-off operation (fuel supply stop) during deceleration.
Next, reference will be made to the operation of the above-mentioned known control apparatus for an internal combustion engine constructed as shown in FIG. 8 by taking, as an example, a case in which failure detection of the EGR system in a deceleration state is carried out.
In this case, first of all, the number of revolutions per minute of the engine Ne is more than or equal to a predetermined value and the throttle valve 7 is in a fully closed state (the idle signal I being in an on state), so a determination is made that the vehicle is in a deceleration state (the failure determination condition being satisfied), and the EGR valve 12 is fully closed to put the engine 1 into a non-EGR operation state, while storing the value of the intake pipe pressure PbOFF at this time.
Subsequently, the EGR valve 12 is forced to open thereby to put the engine into an EGR operation state (EGR gas being introduced), and the value of the intake pipe pressure PbON at this time is stored.
In addition, a pressure difference ΔP between the intake pipe pressure PbON in the presence of EGR and the intake pipe pressure PbOFF in the absence of EGR is calculated according to the following expression (1).ΔP=PbON−PbOFF  (1)
Thereafter, the pressure difference ΔP thus obtained is compared with a predetermined value (fail), which is a lower limit value of a normal pressure difference, and when ΔP≧fail, the pressure difference ΔP is normal (the EGR gas being in a normally introduced state) and hence it is determined that the EGR system is normal.
On the other hand, when ΔP<fail (that is, NO), the pressure difference ΔP has not yet reached the lower limit value of the normal pressure difference (the introduction of the EGR gas being not performed normally), so it is determined that the EGR control section, which constitutes the EGR system, is abnormal.
In general, the intake pipe pressure Pb with the EGR valve 12 being fully closed (in the absence of EGR) is about 35 kPa, whereas the intake pipe pressure Pb when the EGR gas is forcedly introduced with the EGR valve 12 being fully opened (in the presence of EGR) reaches about 60 kPa.
Accordingly, in order to distinguish the normal value (e.g., 25 kPa) of the pressure difference ΔP from abnormal ones, the predetermined value (fail) is set to about 10 kPa, for instance.
Here, reference will be made to the influence on the pressure difference ΔP resulting from a difference between deceleration states such as rapid deceleration and slow deceleration, while referring to a timing chart in FIG. 9 and a characteristic view in FIG. 10.
FIG. 9 is a timing chart that shows the relation between a deceleration flag, an EGR flag (the presence or absence of EGR), the number of revolutions per minute of the engine Ne and the intake pipe pressure Pb as well as the changes thereof over time, while illustrating the operation of the engine when the above-mentioned failure detection operation is carried out during deceleration with the EGR system being normal.
In the curves of the number of revolutions per minute of the engine Ne and the intake pipe pressure Pb in FIG. 9, solid lines represent changes over time in a slow deceleration state, and broken lines represent changes over time in a rapid deceleration state, respectively.
In FIG. 9, intake pipe pressures Pb in the presence and absence of EGR are stored at time points t1, t2 in a failure detection period TA (a valid period in which the failure determination condition is satisfied).
That is, an intake pipe pressure PbOFF1 in the absence of EGR and an intake pipe pressure PbON1 in the presence of EGR are stored at time points t1, t2, respectively, during slow deceleration (solid line) in FIG. 9.
Similarly, during rapid deceleration (broken line), an intake pipe pressure PbOFF2 in the absence of EGR and an intake pipe pressure PbON2 in the presence of EGR are stored at time points t1, t2, respectively.
FIG. 10 is a characteristic view that shows the relation between the number of revolutions per minute of the engine Ne [r/min] and the intake pipe pressure Pb [kPa], in which a solid line represents a case in the absence of EGR, and a broken line represents a case in the presence of EGR, respectively.
In FIG. 10, points b1, b2 on the broken line (a characteristic curve with EGR) and points c1, c2 on the solid line (a characteristic curve without EGR) are shown in association with point a on the solid line before the execution of EGR (a characteristic curve without EGR).
Point b1 on the broken line represents a point that is shifted from the point a on the solid line to the broken line when the change in the number of revolutions per minute of the engine Ne is small, and point c1 on the solid line represents a point on the solid line when the number of revolutions per minute of the engine Ne is equal to the point b1.
Point b2 on the broken line represents a point that is shifted from the point a on the solid line to the broken line when the change in the number of revolutions per minute of the engine Ne is large, and point c2 on the solid line represents a point on the solid line when the number of revolutions per minute of the engine Ne is equal to the point b2.
In FIG. 9, first of all, the decreasing speed of the number of revolutions per minute of the engine Ne is slow or gradual during slow deceleration (see the solid line), and the change in the number of revolutions per minute of the engine Ne in a failure detection period TA is small.
At this time, when EGR is forcedly carried out, the intake pipe pressure Pb shifts from the point a on the characteristic curve of the solid line (in the absence of EGR) in FIG. 10 to the point b1 on the characteristic curve of the broken line (in the presence of EGR), so a pressure difference ΔP (=Pb(b1)−Pb(a)) is obtained.
On the other hand, during rapid deceleration (see the broken line in FIG. 9), the decreasing speed of the number of revolutions per minute of the engine Ne is rapid, and the change in the number of revolutions per minute of the engine Ne in the failure detection period TA is large.
At this time, when EGR is forcedly carried out, the intake pipe pressure Pb shifts from the point a on the characteristic curve of the solid line (in the absence of EGR) in FIG. 10 to the point b2 on the characteristic curve of the broken line (in the presence of EGR), so a pressure difference ΔP′(=Pb(b2)−Pb(a)) is obtained.
Here, note that each of the individual pressure differences ΔP, ΔP′ contains a change component ΔP(EGR) resulting from the execution of EGR and a change component ΔP(Ne) resulting from the change in the number of revolutions per minute of the engine Ne.
Here, it is understood from the characteristic view of FIG. 10 that the curvature of each of the characteristic curves of the solid line (in the absence of EGR) and the broken line (in the presence of EGR) changes in accordance with the number of revolutions per minute of the engine Ne.
In other words, it is found that in the individual pressure differences ΔP (=Pb (b1)−Pb(a)) and ΔP′(=Pb(b2)−Pb(a)) at slow deceleration and rapid deceleration, the change components ΔP(Ne) contained in the individual pressure differences ΔP, ΔP′, respectively, resulting from the number of revolutions per minute of the engine Ne are different from each other even if the degree of EGR opening θe in the presence of EGR is constant.
In other words, the detected value of the pressure difference ΔP might vary depending on the difference of the deceleration state (rapid deceleration or slow deceleration).
Accordingly, when a determination is made as to whether the EGR control device is in failure, by detecting the pressure difference ΔP according to the change in the amount of intake air Qa due to the presence and absence of EGR in the deceleration state, it might become, in the worst case, unable to detect the failure state of the EGR control device, or the normal state thereof might be mistakenly detected as a failure state thereof.
In order to cope with this, in the above-mentioned first patent document, the pressure change index value is further corrected by reading out the change ΔP(Ne) resulting from the change in the number of revolutions per minute of the engine Ne during deceleration as a correction function f based on the number of revolutions per minute of the engine Ne at the time point of detection of the intake pipe pressure Pb.
Next, reference will be made to the operation of controlling the amount of bypass air during deceleration according to the above-mentioned first patent document while referring to a timing chart in FIG. 11, with emphasis being focused on the operation of the bypass air amount control section 9.
FIG. 11 shows the relation among the deceleration flag indicating whether the engine is decelerated or not, the throttle opening θ, and the amount of bypass air Qb together with their changes over time.
In FIG. 11, the solid lines represent characteristics in the case of a throttle opening θA and the alternate long and short dash lines represent characteristics in the case of a throttle opening θB (<θA).
First of all, when the vehicle is traveling with the deceleration flag being “0” (the idle switch 18 being turned off, i.e., the vehicle being not under deceleration), the amount of bypass air Qb is controlled to an amount corresponding to the throttle opening θ.
On the other hand, from a time point t0 at which the deceleration flag is shifted to “1” (the idle switch 18 being turned on, i.e., the vehicle being under deceleration), the amount of bypass air Qb is calculated at predetermined time intervals according to the following expression (2) by using the current amount of bypass air Qbn, the last amount of bypass air Qbn−1 and a predetermined value β.Qbn=Qbn−1−β  (2)
By executing the calculation of the above expression (2), the amount of bypass air Qb is gradually decreasing, as shown in FIG. 11. Such a decreasing operation of the amount of bypass air Qb is referred to as a so-called dashpot operation.
Also, at the time of the operation of the engine load (air conditioner), the electronic control unit 22 outputs an air conditioner control signal D to the air conditioner controller 19A thereby to put the air conditioner in operation, and the bypass air amount control section 9 increases the amount of intake air Qa thereby to ensure the demanded or required output of the engine 1.
Similarly, the bypass air amount control section 9 ensures the demanded engine output by increasing the amount of bypass air Qb in a cooling state of the engine 1.
As a result, even during deceleration (the throttle valve 7 being in the fully closed state), the amount of bypass air Qb might be changed to vary the intake pipe pressure Pb.
Here, reference will be made to an influence on the detected value of the pressure difference ΔP resulting from the change in the amount of bypass air Qb while referring to FIG. 12.
FIG. 12 is a characteristic view when measurements were made in a no-load state and in a complete warm-up state of the engine, respectively, wherein there are shown the relation between the number of revolutions per minute of the engine Ne and the intake pipe pressure Pb in the presence or absence of EGR and the relation between the number of revolutions per minute of the engine Ne and the pressure difference ΔP due to the presence and absence of EGR.
In the relation between the number of revolutions per minute of the engine Ne and the intake pipe pressure Pb in FIG. 9 (see an upper graph), a solid line characteristic curve WA shows the relation between the number of revolutions per minute of the engine Ne and the intake pipe pressure Pb in the absence of EGR with the area of the bypass air passage being α, and a solid line characteristic curve WB shows the relation between the number of revolutions per minute of the engine Ne and the intake pipe pressure Pb in the absence of EGR with the area of the bypass air passage being β (>α).
As is clear from the individual characteristic curves WA, WB, the intake pipe pressure Pb becomes higher in accordance with the decreasing number of revolutions per minute of the engine Ne, whereas the curvatures of the individual characteristic curves WA, WB tend to be different from each other in accordance with the number of revolutions per minute of the engine Ne.
Further, when a comparison is made while focusing on the same number of revolutions per minute of the engine Ne, the intake pipe pressure Pb becomes higher in the case of the characteristic curve WB of the bypass air passage area β(>α) than in the case of the characteristic curve WA of the bypass air passage area α.
In addition, in FIG. 12, a broken line characteristic curve WC shows the relation between the number of revolutions per minute of the engine Ne and the intake pipe pressure Pb in the presence of EGR (the degree of EGR opening θe being constant) with the area of the bypass air passage being α, and a solid line characteristic curve WD shows the relation between the number of revolutions per minute of the engine Ne and the intake pipe pressure Pb in the presence of EGR (the degree of EGR opening θe being constant) with the area of the bypass air passage being β.
The characteristic curves WC, WD in the presence of EGR (broken lines) are higher in the intake pipe pressure Pb than the characteristic curves WA, WB in the absence of EGR (solid lines).
Moreover, as is clear from the characteristic curves WC, WD, similar to the case in the absence of EGR (solid lines), the lower the number of revolutions per minute of the engine Ne, and the larger the bypass air passage area, higher does the intake pipe pressure Pb become, so the curvature is varied in accordance with the number of revolutions per minute of the engine Ne.
On the other hand, in the relation between the number of revolutions per minute of the engine Ne and the pressure difference ΔP due to the presence and absence of EGR in FIG. 12 (see a lower graph), a solid line characteristic curve WE shows the relation between the number of revolutions per minute of the engine Ne with the area of the bypass air passage being α and the pressure difference ΔP (characteristic curve WC-WA) due to the presence and absence of EGR, and an alternate long and two short dashes line characteristic curve WF shows the relation between the number of revolutions per minute of the engine Ne with the area of the bypass air passage being β and the pressure difference ΔP (characteristic curve WD-WB) due to the presence and absence of EGR.
In the characteristic curves WA through WD (for the relation between the number of revolutions per minute of the engine Ne and the intake pipe pressure Pb), the curvatures (for characteristic curves WA, WC and characteristic curves WB, WD) with respect to the number of revolutions per minute of the engine Ne are different from one another according to the presence or absence of EGR, so in the characteristic curves WE, WF (for the relation between the number of revolutions per minute of the engine Ne and the pressure difference ΔP), the lower the number of revolutions per minute of the engine Ne, the larger does the pressure difference ΔP become.
Further, the curvatures of the characteristic curves WA through WD (the characteristic curves WA, WB and the characteristic curves WC, WD) with respect to the number of revolutions per minute of the engine Ne are different from one another according to the magnitude (α<β) of the bypass air passage areas α, β, so the characteristic curves WE, WF (the relation between the number of revolutions per minute of the engine Ne and the pressure difference ΔP) are different from each other according to the magnitude of the bypass air passage areas α, β.
In FIG. 12, a solid line E (the bypass air passage area α) represents a characteristic curve when a target amount of bypass air is fixed to 115 [L(liter)/min], and an alternate long and two short dashes line F (the bypass air passage area β) represents a characteristic curve when the target amount of bypass air is fixed to 260 [L/min].
In the relation between the number of revolutions per minute of the engine Ne and the pressure difference ΔP, the pressure difference ΔP becomes larger by about 1 kPa when the target amount of bypass air is fixed to 260 [L/min] (the alternate long and two short dashes line F) than when the target amount of bypass air is fixed to 115 [L/min] (the solid line E).
Thus, when the area of the bypass air passage (the amount of bypass air) varies, a difference is generated in the pressure difference ΔP between the intake pipe pressures Pb in the presence and absence of EGR.
Accordingly, even if the change ΔP(Ne) according to the change in the number of revolutions per minute of the engine Ne at the time of deceleration is corrected based on the number of revolutions per minute of the engine Ne at the time point of detection of the intake pipe pressure Pb, there is a possibility that the detected value of the pressure difference ΔP might be varied when the intake pipe pressure Pb varies resulting from the change in the amount of bypass air Qb due to the bypass air amount control section 9, as stated above.
As a result, in a device that makes a failure determination based on the amount of change of the intake pipe pressure Pb (the pressure difference ΔP) according to the change in the amount of intake air Qa due to the presence and absence of EGR, there is a possibility, in the worst case, of becoming unable to detect a failure state or of mis-detecting a normal state as a failure state.
Accordingly, in the above-mentioned second patent document, the intake pipe pressure Pb detected upon the forced opening and closing of the EGR valve is corrected by using the number of revolutions per minute of the engine Ne and the amount of bypass air Qb detected upon the forced opening and closing of the EGR valve, and an EGR rate equivalent value PEGR (corresponding to the flow rate of EGR Qe) is calculated from the intake pipe pressure thus corrected, whereby a failure of the EGR control device is detected based on the EGR rate equivalent value PEGR.
Hereinafter, reference will be made to the correction processing using the number of revolutions per minute of the engine Ne and the amount of bypass air Qb as well as the failure detection processing using the EGR rate equivalent value PEGR according to the above-mentioned second patent document.
In this case, in comparison with the processing section of the above-mentioned first patent document, there are added a correction section for correcting the intake pipe pressure Pb in accordance with the change in the number of revolutions per minute of the engine Ne, a correction section for correcting the intake pipe pressure Pb in accordance with the change in the amount of bypass air Qb, a calculation section for calculating the EGR rate equivalent value PEGR, and an error suppression section for suppressing an error of the intake pipe pressure Pb based on the EGR rate equivalent value PEGR.
Here, note that in order to compensate for the influence due to an increased amount of bypass air Qb in the cooling state of the engine 1, a failure determination is not executed in a temperature range in which the amount of bypass air Qb increases (e.g., the cooling water temperature Tw being equal to or lower than 80° C.).
First of all, when it is determined that the engine 1 is under deceleration, the EGR valve 12 is fully closed to put the engine 1 into a non-EGR operation state, and the values of the intake pipe pressure PbOFF, the number of revolutions per minute of the engine NeOFF and the amount of bypass air QbOFF at this time are stored.
Subsequently, the EGR valve 12 is fully opened to put the engine 1 into an EGR operation state (the EGR gas being introduced), and the value of the intake pipe pressure PbON and the number of revolutions per minute of the engine NeOFF at this time are stored.
Then, a pressure difference after correction (hereinafter referred to as a corrected pressure difference) ΔPf is calculated by using the pressure difference ΔP between the intake pipe pressure PbON in the presence of EGR and the intake pipe pressure PbOFF in the absence of EGR, and the correction function f based on the numbers of revolutions per minute of the engine NeON, NeOFF in the presence and absence of EGR, as shown by the following expression (3).ΔPf=ΔP−{f(NeON)−f(NeOFF)}  (3)
Subsequently, the EGR rate equivalent value PEGR is calculated by using the corrected pressure difference ΔPf calculated from expression (3) above and the intake pipe pressure PbOFF in the absence of EGR.
At this time, the EGR rate equivalent value PEGR is calculated by using the intake pipe pressure corrected by a correction function g based on the number of revolutions per minute of the engine NeOFF and the amount of bypass air QbOFF in the absence of EGR, as shown by the following expression (4).PEGR=[ΔPf/{PbOFF−g(NeOFF,QbOFF)}]×100  (4)
By correcting an error in the EGR rate equivalent value PEGR due to the change in the amount of bypass air Qb according to the above-mentioned processing, reliability in the failure detection of the EGR control device is improved.
Thereafter, normality or abnormality of the EGR system is determined based on whether the EGR rate equivalent value PEGR is higher than or equal to a threshold value PEGR(fail).
As described above, in the known EGR system failure detection device disclosed in the above-mentioned first patent document, there is used the correction section that reads the error of the pressure difference ΔP, which results from the change in the number of revolutions per minute of the engine Ne due to the difference of the deceleration state, as the correction function f based on the number of revolutions per minute of the engine Ne at the time point of detection of the intake pipe pressure Pb, but the influence of the error of the intake pipe pressure Pb resulting from the difference of the amount of bypass air Qb at the time point of detection of the intake pipe pressure Pb is not taken into consideration, so there has been a problem that proper correction might not be able to be made.
In addition, in the known device disclosed in the above-mentioned second patent document, the correction section is used that reads the error of the intake pipe pressure Pb, which results from the difference of the amount of bypass air Qb, as the correction function g based on the number of revolutions per minute of the engine Ne and the amount of bypass air Qb, but the amount of bypass air Qb is estimated by the use of a map that has been set beforehand, so when there are tolerances of component parts or there occurs a change over time thereof due to the influence of deposits, etc., the actual amount of bypass air becomes different from the amount of bypass air Qb that is estimated from the map set beforehand, and in this case, too, there is still a problem that proper correction might not be made.
Also, in the known device disclosed in the above-mentioned second patent document, a correction due to the correction function g is executed with respect to only the intake pipe pressure PbOFF in the absence of EGR, but at the time of EGR introduction, the EGR gas is introduced from the exhaust pipe 15 into the intake pipe 3 under the action of a differential pressure between an upstream pressure (in the exhaust pipe 15) upstream of the EGR valve 12 and a downstream pressure (the intake pipe pressure Pb) downstream of the EGR valve 12. As a result, there is the following problem. That is, when the amount of bypass air Qb (the area of the bypass air passage) is varied, not only the characteristic curves WA, WB for the relation between the intake pipe pressure Pb and the number of revolutions per minute of the engine Ne in the absence of EGR (see FIG. 12) but also the characteristic curves WC, WD for the relation between the intake pipe pressure Pb and the number of revolutions per minute of the engine Ne in the presence of EGR are also changed, in spite of which it is impossible to make proper correction with respect to the pressure difference ΔP due to the presence and absence of EGR.
Moreover, as a consequence of this, there is also another problem that in case where a failure determination is made based on the amount of change of the intake pipe pressure Pb (the pressure difference ΔP) according to the change in the amount of intake air Qa due to the presence and absence of EGR, there is a possibility, in the worst case, that a failure state can not be detected or a normal state may be mis-detected as a failure state.
Further, in the known device disclosed in the above-mentioned second patent document, a failure determination is not carried out in a temperature range in which the amount of bypass air Qb is increased (e.g., the cooling water temperature Tw being equal to 80° C. or below), so there is a further problem that the temperature region where failure detection processing (failure diagnosis) is executed is limited.